The present invention relates to improved refractories and moulds for high temperature metal casting, methods of making the same, and improved processes for casting high melting alloys, such as steel and iron alloys. The invention is particularly concerned with the ferrous casting industry where the vast majority of parts are made by sand casting.
Quartz sand has always been the principal refractory used by ferrous foundries for sand moulds and cores. When the quartz grains have a high purity, they have enough refractoriness to permit ferrous metal casting. However, crystalline silica or quartz is an inferior refractory because its refractoriness is much lower than other refractories, such as zircon and alumina, and because it has notoriously poor thermal shock resistance due to the sudden alpha-beta inversion (.alpha..fwdarw..beta.) as the crystalline silica converts from the alpha to the beta form or vice versa (e.g., beta-quartz to alpha-quartz).
Thermal shock is less of a problem in standard sand moulds and sand cores used in ferrous foundries because of high permeability, the absence of substantial amounts of fine refractory particles between the sand grains, and the large size of the grains (e.g., 150 to 200 microns). The large spaces between the sand grains provide room for expansion and contraction of individual grains and thereby minimize the total forces generated during the alpha-beta inversions. This is not true, however, in compacted silica moulds of low permeability having substantial amounts of rigid refractory material occupying the spaces between the refractory grains, and the sudden volume change during the alpha-beta inversion may cause defective castings due to mould failure, spalling, cracking and the like.
In spite of its shortcomings, quartz sand was, at one time, commonly used in the precision investment casting industry to make precision moulds and cores. Prior to 1955, quartz sand was used commercially for precision investment casting of small metal parts by the "lost-wax" process. Rigid metal flasks were used to receive the refractory moulding composition and to provide the mould with the needed strength. The foundry procedures had to be carefully controlled to avoid catastrophic damage to the mould due to the alpha-beta inversion problem, and the problem was even more serious when cores were used. The quartz moulds required slow heating through the alpha-beta inversion to minimize damage due to the sudden expansion of the crystalline silica particles. After the mould temperature was above 400.degree. C., the inversion was no longer a problem and the mould was fired to eliminate combustibles and then used, while still hot, to cast the metal (e.g., a turbine blade). The metal had to be poured while the mould was still above the alpha-beta inversion temperature to avoid mould shattering due to the violent expansion during contact of the mould with the molten metal and to avoid damage from two more trips through the crystallographic inversion (e.g., contraction damage during cooling to room temperature and expansion damage during subsequent heating). Even though the damage to the quartz mould (or core) could be reduced by cooling and heating the mould very slowly over a very long period of time, such precautions did not solve the problem and were too costly to be practical. Thus, prior to 1955, it was important for each metal foundry engaged in precision investment casting to have equipment for firing its own silica moulds and cores.
Soon after 1955, these foundries became obsolete. With the advent of flash-fire dewaxing, the precision investment casting industry turned away from quartz sand moulds and moulding flasks and used shell moulds instead. It became impractical to use quartz sand or crystalline quartz in precision ceramic cores, and core manufacturers found it necessary to employ large amounts of vitreous silica to provide proper thermal shock properties in precision investment cores and to minimize the alpha-beta inversion problem. For the last twenty years the typical precision cores for investment casting of superalloys have contained 60 to 80 percent by weight of vitreous silica and 20 to 40 percent by weight of zircon or other refractory.
It was necessary to use major amounts of silica in the cores to facilitate leaching, and vitreous silica was preferred because of its excellent thermal shock properties in spite of the potential devitrification problems and inferior refractoriness. Typical cores containing 70 percent silica and 30 percent zircon have adequate refractoriness to permit casting of common nickel-base and cobalt-base alloys used in aircraft turbine engine parts, for example, and have been used for this purpose for two decades.
Although it has poor leaching characteristics, zircon is a superior refractory. In a core the amount used is preferably no more than 30 to 40 percent by weight, but in a precision investment shell mould much more can be used because leaching is not necessary. Zircon has better refractoriness than silica and, unlike silica, is not subject to the sudden catastrophic volume changes characteristic of the alpha-beta inversion. During the last 20 years, precision investment shell moulds have employed major amounts of refractories, such as zircon, aluminum silicate or alumina, which permitted casting of nickel-base and cobalt-base superalloys at temperatures up to 1550.degree. C. or higher. Shell moulds of this type could also be used to cast steel alloys because of the excellent high temperature properties. The metal casting temperatures had to be limited, however, when using leachable cores containing major amounts of silica in order to avoid core failure or excessive core deformation. Heretofore, the inferior performance of typical silica cores at high temperatures made them a poor choice for casting metals, such as steels and superalloys, which required unusually high pouring temperatures. This is one reason that the typical precision ceramic cores used in investment casting have heretofore been considered by the ferrous casting industry as generally impractical for commercial casting of steel and iron alloys.
The limitations of known refractories have always provided a serious problem when casting ferrous alloys because of the high casting temperatures required, and ferrous foundries have long needed an improved refractory system, particularly one which would permit casting at reasonable cost with greater precision and fewer casting defects. As previously pointed out, crystalline silica or quartz sand has inferior refractoriness and is particularly troublesome because of the alpha-beta inversion, but quartz sand is still the mainstay of the ferrous casting industry, both for moulds and cores. Refractories with superior refractoriness, such as alumina, zirconia and magnesia, are generally not used for ferrous casting because of poor thermal shock characteristics. High purity quartz sand is by far the preferred refractory for sand casting, although zircon and olivine sands are also used. Fused silica sand or vitreous silica has heretofore been considered unsatisfactory by ferrous foundries as a replacement for quartz sand in sand moulds and cores, although it has sometimes been used in small amounts to improve the high temperature strength of sand cores.
Because the present invention is concerned with a simple, but revolutionary change in the refractory systems used for casting of ferrous alloys and solves problems which have existed for many decades, it is important, in order to understand the nature of the problems, to consider the conventional processes used by ferrous foundries.
Basic casting processes used for non-ferrous metals, such as permanent-mould casting and die casting, are not practical for casting ferrous alloys. Sand casting has been the only process considered suitable for commercial foundry casting of ferrous alloys because of the extremely high pouring temperatures required, often exceeding 1600.degree. C. and this process accounts for the vast majority of all metal castings.
Sand casting processes employ a low-temperature binder or organic binder which is present when the molten metal is poured into the sand mould and provides the mould with strength until the metal solidifies. A molten ferrous alloy is poured into the sand mould, usually while the mould is at room temperature, and causes the mould to heat rapidly so as to weaken or destroy the organic binder near the metal-mould interface, but the mould usually has a large mass, several times that of the metal casting and can absorb large amounts of heat to avoid overheating so that the mould will hold together until the metal solidifies.
Foundries collect and reuse the moulding sand because it requires about 4 to 5 tons of sand for each ton of metal casting. When cores are used, they are commonly made from the same type of sand as the mould to minimize contamination of the moulding sand.
A moulding sand may contain up to 50 percent by weight of clays, such as bentonite or fire clays. When additional strength is needed, a sand mould or core is made of a core sand containing an organic or resin binder which can be hardened by baking at a temperature of 250.degree. to 450.degree. C.
The sand used in making the mould or core must provide good refractories to withstand the high pouring temperatures, high permeability to permit rapid escape of gases, and collapsibility to permit the metal to shrink after it solidifies. It should also have thermal shock resistance because heat from the molten metal causes rapid expansion of the sand surface at the mould-metal interface which can result in spalling, cracking, buckling or flaking at the mould surface.
Sand casting is a terribly demanding application for a refractory mould body, particularly a core body, because of the extremely severe conditions involved in casting ferrous alloys. Thermal shock, for example, is at a maximum when refractory grains at room temperature engage molten iron at 1500.degree. C. to 1600.degree. C. A core body with small mass is subject to thermal shock damage due to the extremely rapid heating of the entire body and also to deformation or failure due to insufficient refractoriness, strength and resistance to viscous flow. The larger sand cores present less of a problem because they have adequate mass to avoid overheating before metal solidification and they are adequately strengthened by the organic binders in the cooler parts of the core, but the problems become extremely serious as the size and mass of the core is reduced.
For many decades the ferrous casting industry has had extreme difficulty in producing long holes of small diameter because of inadequate core technology and the limitations of known refractory materials. A small core used for ferrous casting can heat up above 1500.degree. C., before the metal solidifies if the casting has significant mass, and the result is that the core loses strength, is unable to resist the large buoyant and inertia forces exerted by the molten metal on the core, and breaks or deforms. For this reason it has heretofore been impractical to provide thick steel castings with cored holes having small diameters, such as 1 to 2 centimeters, and length-to-diameter ratios in excess of 10:1, for example.
This problem has been recognized for many decades; but, heretofore, a satisfactory solution was not found. Deformation and sagging of the longer core can be reduced by employing wire reinforcement or metal chaplets, but these are undesirable and are avoided as much as possible. Wires interfere with removal of the cores from the casting, can become stuck in or welded to the casting, and makes the cores more expensive to make. Chaplets are difficult to place, are unreliable and greatly reduce the rate of production while adding excessive cost. They also reduce the quality of the casting because they become part of the casting.
Attempts have been made to improve the hot strength of cores used in ferrous casting. In an oil-cereal-bonded sand core, for example, the organic binders char or carbonize at 400.degree. to 500.degree. C. and a coke bond may develop over the range of 400.degree. to 1000.degree. C. Supplementary binders such as bentonite, fire clay and iron oxide have been used to improve the strength of the core at temperatures above 1000.degree. C. For example, a core may have a strength of ten kilograms per square centimeter at 1400.degree. C. but the strength may drop to less than one kilogram per square centimeter at 1500.degree. C. Heretofore, special cores have been used by ferrous foundries when high temperature strength became critical, but these generally had a strength less than 25 kilograms per square centimeter at 1400.degree. C. At 1500.degree. C. such a core could have insufficient strength to avoid deformation or failure due to the buoyant force of the molten metal.
Because of the limited high temperature strength and refractoriness of known core materials, the ferrous casting industry has adopted certain recommended limiting dimensional relationships for cores. The industry recommends that cores and isolated mould elements have a thickness at least twice the thickness of the surrounding metal sections and that cylindrical holes formed by cores with a diameter less than twice the metal wall thickness have a length no greater than the diameter. If the cylindrical hole formed by the core has a diameter from two to three times the thickness of the surrounding metal sections, then the accepted industry recommendation is that the length be no more than three times the diameter. For blind holes formed by cores supported at one end only, the recommended length is fifty percent less. Longer sand cores can be used if the metal thickness is reduced to permit more rapid solidification, but it is usually important to provide castings with substantial thickness.
The casting conditions and size limitations on cores are, of course, quite different in the precision investment casting industry when casting aircraft turbine engine components and the like from nickel-base and cobalt-base alloys. Precision leachable ceramic cores of the type used for investment casting during the last 20 years and made from major amounts of fused silica are entirely different from sand cores and have generally been considered impractical for commercial sand casting of steel and iron alloys. A typical sand core is made from sand grains having a substantial size, such as 150 to 200 microns and higher, and relatively free of fine particles so as to provide an AFS permeability number of 70 to 200, which is needed to avoid serious gassing problems, whereas a typical precision core for investment casting has an average particle size below 20 microns and a permeability too low for assigning a permeability number. A typical precision investment core has low permeability, loses strength at high temperatures, has inadequate strength at 1500.degree. C. to avoid substantial deformation, and should not be used for casting ferrous alloys. Precision cores of the type used for investment casting are also inappropriate for use by ferrous foundries because of excessive cost and the difficulty of removing the core from the casting. The cost of precision investment cores is many times that of sand cores of comparable size. Sand cores also have the advantage of collapsibility and the ability to break down during knockout and cleaning operations. Precision investment cores, on the other hand, are much more difficult to remove from the metal casting and require leaching.
Standard precision silica cores of the type made in the investment casting industry during the last two decades for example, by the extrusion process, the injection moulding process or the ethyl silicate process are suitable for casting non-ferrous metals but are inappropriate for ferrous casting and will produce serious casting defects. The rapid heating of such a core from room temperature to 1500.degree. C. or higher due to contact with molten steel, is accompanied by a sudden gas expansion that will produce gas holes and other surface defects in the casting. If such a standard silica core were to be used in a sand mould for making an internal cavity in a steel casting, the surface of such cavity would be rough and irregular and would contain blow holes or similar defects, making the casting surface undesirable and making core removal more difficult.
The vast majority of lost-wax investment castings are produced in thin-wall shell moulds built up by repeated applications of slurry dip coats and coarse stucco layers using an ethyl silicate binder, a colloidal silica binder or other high temperature binder in the ceramic slurry, but cope-and-drag investment flasks of the type commonly used prior to 1950 are sometimes employed for lost-wax investments. For example, quartz sand and/or other refractory material plus a suitable high temperature binder may be placed in the flask and compacted around the wax pattern, the pattern removed, and the mould section fired at a temperature of 1000.degree. C. to 1100.degree. C. to eliminate combustibles and form a rigid structure before the mould is used for metal casting.
The refractory mix used to fill the investment flask sometimes employs an ethyl silicate binder system in accordance with the so-called "Shaw process" which is relatively costly but is used to a limited extent. In that process, a permanent siliceous bond is produced by the formation of a silica gel in a hydrolyzed solution of the ester. For example, block moulds or contoured shells can be produced in the Shaw process using the gelation of ethyl silicate to bond a graded refractory, such as sillimanite. The hydrolyzed suspension is poured around the pattern and then sets to a rubbery consistency, at which stage the pattern can be withdrawn. To assist drying, the alcohol vapor is ignited with gas burners and the volume shrinkage of the investment produces craze cracking before the investment flask is placed in the furnace for firing to remove final traces of water and alcohol. The "microcrazed" structure comprises a multiplicity of craze cracks scattered throughout the mould body which weaken the mould but improve the permeability and thermal shock properties. When the crazed mould body is exposed to the heat shock of oven firing or contact with molten metal, the craze cracks close to move slightly to accept expansion of refractory grains and minimize or avoid rupture forces between grains. Ethyl silicate investment moulds therefore have good permeability and good thermal shock properties when using refractories with high refractoriness, such as zircon or sillimanite, and it is possible to cast ferrous alloys as well as nickel-base and cobalt-base superalloys; but, the excessive cost of such investments and other disadvantages limit their utility.
A number of practical problems arise when an ethyl silicate binder system is employed in the manufacture of refractory cores. The extensive network of craze cracks with lengths often several times the width of the refractory grains makes it possible to obtain permeabilities ten times that of conventional cores using other binders, but the craze cracks tend to reduce the strength of the core body and its resistance to deformation or failure. The cost of making ethyl silicate cores is many times that of other cores because of the low rate of production. Only a minor proportion of the precision cores used for lost-wax investment castings use the ethyl silicate binder system.
A typical precision ceramic core as used for forming internal cavities in turbine blades and other aircraft turbine engine components is made by a mass production process, such as extrusion or injection moulding, and can produce internal cavities with a high degree of accuracy and with a good surface finish, especially when the core is machined to close tolerances. Such precision has been achieved in the lost-wax investment casting industry for two decades, and porous leachable silica cores have made it possible to produce precision cavities of small diameter in the metal castings without the need for subsequent machining of the metal. Although conventional silica cores are temperature limited and will bow or deform if the casting temperature is excessive, they can produce long small diameter holes in most non-ferrous castings with a high degree of precision. This is not the case in the ferrous casting industry. Ferrous foundries have heretofore had no practical economical way to produce long small diameter holes in sand castings with close dimensional tolerances and have, therefore, found it necessary to rely on subsequent drilling or machining operations.
Precision ceramic cores based on silica have heretofore been inappropriate for ferrous casting and have failed to provide a satisfactory solution to this problem because of the thermal shock problem, gassing problems, and the excessive casting temperatures needed. Silica is a poor choice as a refractory for such high temperature casting. Vitreous silica has thermal shock resistance but cannot maintain its shape at 1500.degree. C. and above. Maximum thermal shock resistance is needed for a refractory core heated in seconds from room temperature to 1500.degree. C. by a molten iron alloy, and a crystalline silica is very poor in this respect.
Cristobalite, for example, has notoriously poor thermal shock properties due to the alpha-beta inversion. Even though its refractoriness is superior to that of vitreous silica, cristobalite has generally been considered undesirable and has seldom been added in substantial amounts to a refractory moulding composition except in custom-fitted dental moulding operations to compensate for contraction of the metal casting. It is possible, however, to heat a mould or core containing more than 60 percent by weight of alpha cristobalite from room temperature to 1500.degree. C. without shattering the mould if the heating is very slow through the alpha-beta inversion and the mould is highly porous and can accommodate expansion of the individual grains. Damage by thermal shock can thereafter be minimized by pouring the molten metal after the mould or core is fired and heated to a high temperature, such as 1400.degree. C. to 1500.degree. C. There will be less damage due to sudden volume changes during the alpha-beta inversion if the silica mould contains less than 50 percent by weight of cristobalite at room temperature and is devitrified during firing to provide much larger amounts of cristobalite, provided that the mould is not cooled before metal casting. The slow heating and firing in the same foundry where the metal is cast is uneconomical and generally impractical, but this procedure can be employed to facilitate removal of a devitrified silica shell mould from a metal casting. For example, it has been proposed that a silica shell mould should be provided with large amounts of cristobalite formed by a mineralizer during firing so that after casting, the mould can be rapidly cooled with the metal casting below the alpha-beta inversion temperature to shatter the mould (see U.S. Pat. No. 3,540,519).
In the directional solidification of modern superalloys, all-silica cores can be devitrified just prior to metal casting by preheating and firing them up to one hour at a temperature of 1400.degree. C. or higher to provide over 90 percent by weight of cristobalite before the metal is poured into the preheated mould (see U.S. Pat. No. 4,093,017). However, such a process is not applicable to sand casting of ferrous alloys and of no practical value to ferrous foundries, which do not prefire the sand moulds. Cristobalite heretofore has been considered inappropriate as a refractory for ferrous casting, particularly because of the severe thermal shock problems involved in sand casting operations.
In a large sand mould or sand core, thermal shock is less of a problem because of the very high permeability and the large spaces between grains to accommodate sudden expansion of the grains. Nevertheless, cristobalite is not considered desirable in a sand mould or core. In a conventional precision core made by extrusion or injection moulding, the silica particles are tightly compacted under high pressure, the permeability is a very small fraction of that of a sand core, and there is little room for the silica particles to expand. As a consequence, thermal shock problems are much more severe when casting at excessive temperatures. Silica cores of this type therefore require the superior thermal shock properties of vitreous silica and suffer from the inherent disadvantage of viscous flow at high temperature. The low permeability and low high temperature strength characteristic of these cores made them inappropriate for the standard sand casting processes, and the ferrous casting industry therefore did not use precision silica cores in such processes. Instead the industry continued to struggle with the existing core technology and proceeded on the assumption that there was no simple economical way to effect precision casting of long internal cavities of small cross section.